1. Field
The present document relates generally to wireless communication, and amongst other things to, signal transmission in multi-antenna system.
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels may also be referred to as a spatial subchannel (or a transmission channel) of the MIMO channel and corresponds to a dimension.
If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on the other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases
To further improve the diversity of the channels a transmit diversity technique may be utilized. Many transmit diversity techniques have been explored. One such technique is transmit delay diversity. In transmit delay diversity a transmitter utilizes two antennas that transmit the same signal, with the second antenna transmitting a delayed replica of that transmitted by the first antenna. By so doing, the second antenna creates diversity by establishing a second set of independent multipath elements that may be collected at the receiver. If the multipath generated by the first transmitter fades, the multipath generated by the second transmitter may not, in which case an acceptable Signal-To-Noise Ratio (SNR) will be maintained at the receiver. This technique is easy to implement, because only the composite TX0+TX1 channel is estimated at the receiver. The biggest drawback to transmit delay diversity is that it increases the effective delay spread of the channel, and can perform poorly when the multipath introduced by the second antenna falls upon, and interacts destructively with, the multipath of the first antenna, thereby reducing the overall level of diversity.
To deal with standard delay diversity problems, additional delay diversity techniques have been developed. One such technique is referred to as cyclic delay diversity. A cyclic delay is one where the samples of each symbol of the ni symbols are shifted in the order in which they are transmitted as part of the symbol. Those samples that are beyond the effective part of the symbol are transmitted in the beginning of that symbol. In such a technique, a prefix is pre-pended to each sample that fixes a delay, or order, for transmitting the sample from the specific antenna as part of the symbol. The cyclic delays allow for longer delays, however, which would otherwise be limited to fractions of the guard interval period to avoid inter-symbol interference.
A cyclic delay diversity scheme may introduce frequency selectivity in the channel and hence may provide diversity benefit for flat channels. It does not provide, however, any time diversity when the channel is not in and of itself time selective. For example, if two transmit antennas are in slow fading or static channels, the cyclic shift Δm may be such that the two channels, e.g. H1(n) and H2(n), add up destructively (or constructively) all the time.
Therefore, it is desired to provide a delay diversity scheme which minimizes the possibility of destructive or constructive addition of the channels utilized to provide diversity.